I've seen questions like this one that have no coding implications per se, but revolve around Apple's HIG and as such can only be answered by developers. Are these off topic or not? Why?
Have you checked our User Interface Stack Exchange site??
Seems like a perfect fit if you ask me!
I consider questions that ask about whether the specific use of a control would violate the Human Interface Guidelines to be on topic for Stack Overflow. These tend to be different from your normal questions that ask about ways of improving the user experience, because there are definitive rules that guide their usage, and there's normally a specific solution to the issue that requires code.
The Human Interface Guidelines also shape my answers to questions. If someone asks how to do something in code that will violate the guidelines (like changing the color of alert views), I'm more likely to answer by telling them not to do that rather than just blindly giving them code that I know will cause trouble for them later on.
More subjective questions on user experience might be good candidates for the User Interface Stack Exchange site, but I think these specific HIG-related ones that affect your application design belong here.